This invention relates generally to the field of failure analysis of integrated circuits, and more particularly, this invention relates to the detection and imaging of internal magnetic fields surrounding currents within operating integrated circuits. Because integrated circuits (ICs) are the key components for nearly all technological systems, design verification, model validation, and analysis of failures of ICs are necessary to ensure their quality and reliability. Complete and detailed IC analysis requires that internal conductor voltages and currents be determined during operation. But, as ICs have become more complex and the sizes of features on the ICs continue to shrink to sub-micrometer dimensions, these tasks have become increasingly difficult. Although techniques for voltage measurement on IC conductors have been available for some time, no practical techniques exist to determine IC currents. Detecting current magnitude, direction or phase, and waveform in ICs is especially desirable to verify design, to analyze analog circuits wherein the currents carry information, and to analyze IC failures when the only signature of failure is anomalous current with no other detectable attribute. Development of a sensitive, non-invasive method to determine currents in internal conductors on ICs is a critical enabling technology for next-generation IC analysis.
Previously, current in IC conductors has been measured by mechanical probing of devices or by inferring the presence of current from liquid crystal or light emission experiments. The electrical probing technique, however, is destructive and inaccurate because of contact resistance effects. Moreover, liquid-crystal and light-emission techniques provide only qualitative data. The first attempt to noninvasively detect current in ICs was done by Helmreich et al. in 1991 who used asymmetric secondary electron emission around current-carrying conductors in a scanning electron microscope and detected 100 mA ac currents on IC test structures. However, low sensitivity and voltage-contrast effects limit the usefulness of this technique.
Magnetic force microscopy is one of several scanning probe microscopy (SPM) techniques. Herein the generic term scanning probe microscopy refers to scanning force microscopy and techniques derived from scanning force microscopy. Samples are imaged in a scanning probe microscope by scanning a sharp probe tip attached to a cantilever close to the sample surface.
A scanning probe microscope has two operating modes: contact and non-contact. In contact mode scanning probe microscopy, the tip is close enough to the surface that a repulsive interaction occurs between the atoms in the tip and in the surface which is detected by monitoring the deflection of the cantilever. A feedback mechanism maintains this repulsive force at a constant magnitude by changing the tip-sample distance. This change develops a topographic image, i.e., a constant force image, of the sample. Piezoelectric elements, capable of producing displacement as small as 0.01 nanometers (nm) are used for positional control of the tip or sample in the x, y, and z directions. Spatial resolution can be achieved on the order of 0.1 nm.
In non-contact mode scanning probe microscopy, the tip is moved 10 nm to 500 nm away from the sample surface, and some spatial resolution is lost in this mode because the tip is farther from the sample surface. Whereas, in the contact mode imaging the atomic repulsive force fields dominate the interaction between tip and sample, in the non-contact mode, the longer-range interactions, such as Van der Waals, magnetic, and electrostatic forces, become important. The sensor or tip must be magnetic in order to interact with the field of interest, i.e., to detect magnetic field gradients. The scanning probe microscopy image is a composite of the effects of all forces acting on the tip. In the absence of other field gradients, long-range Van der Waals forces attract the tip to the sample surface and can be used to generate a topographic image of the surface. Magnetic field gradients can be imaged if the scanning probe microscope tip has sufficient magnetic dipole moment. Depending on the relative strength of the magnetic field and Van der Waals gradients as well as the characteristics of the magnetic tip, the non-contact mode image may show only the magnetic field effects, a superposition of magnetic and topographic effects, or only topography.
In non-contact mode scanning probe microscopy, the tip vibrates perpendicular to the sample surface with an amplitude of approximately 1 nm to prevent attractive forces from drawing the tip into contact with the surface. Tip vibration results from oscillating the cantilever that supports the tip at or near its resonant frequency with an additional piezoelectric element. The local force gradients parallel to the direction of tip vibration (dF.sub.z /dz) interact with the vibrating tip and modify the effective spring constant, C, of the cantilever according to C=C.sub.o +dF.sub.z /dz, where C.sub.o is the spring constant of the isolated cantilever. If the interaction is attractive, the cantilever will effectively soften, and the resonant frequency will decrease. Conversely, a repulsive interaction will increase the resonant frequency. Changes in the interaction force may be detected by monitoring the amplitude, phase, or frequency of cantilever vibration.
Magnetic force microscopy was developed as a probe to image magnetic fields and domains in magnetic thin films and recording media, and spatial resolution of magnetic features approaching 10 nm have been achieved. Thus, magnetic force microscopy has been shown to be an effective tool to measure small magnetic fields arising from submicrometer scaled features. Three-dimensional simulations and experimental detection of the magnetic field surrounding a current driven single straight conductor were performed by Goddenhenrich et al. in 1990 for purposes of calibrating a magnetic force microscope tip to determine bit structures of data storage media and complete thin-film magnetic devices. But to date, no one has suggested or hinted that the induced magnetic fields from internal currents in integrated circuits can be used to detect and image those currents.
Another non-contact scanning force microscopy technique is charge force microscopy in which electrostatic forces between the tip and sample are measured. Charge force microscopy is used to detect electric fields such as those associated with electrical potentials on conductors. In one variation, the scanning tip is a conductor to which a voltage is applied, i.e., the local electric field gradients exert a force on the conductive tip which is held at a particular voltage. Charge force microscopy has been used to detect and image voltages on IC conductors, and has been used by Hou et al. in 1992 and by Bloom in 1994 to obtain gigahertz waveforms by using a mixing technique. The sensitivity of charge force microscopy is approximately 1 mV. This is as good as, but not an improvement over, the sensitivity of other techniques, principally electron beam techniques, used for detecting voltages and voltage waveforms on operating ICs.